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Editors Selection IGR 13-4
Examination methods: IOP: GAT calibration
Comment by James Brandt on:
18004 Goldmann applanation tonometer calibration error checks: current practice in the UK, Kumar N; Jivan S, Eye, 2007; 21: 733-734
A variety of studies suggest that under ideal clinical conditions (i.e., during the course of a controlled research trial), intra-observer reproducibility of GAT is about ± 2.5 mmHg; inter-observer reproducibility is lower. Out in the 'real world', reproducibility is even worse than this, making any single IOP estimate just that - an estimate. Nonetheless, GAT remains clinically useful and widely deployed in clinical practice - GAT isn't going to be replaced any time soon. Therefore it is worthwhile to re-visit all potential sources of error and try to reduce them in clinical practice.
The GAT is a remarkably simple device that can go out of calibration due to corrosion, gumming of lubricants and damage to the internal mechanism.
Before we try to interpret our GAT measurements up or down a few mmHg because of thick or thin corneas, we should at least make sure that the underlying measurement is soundThe most common cause of calibration failure is the mounting of a wet tonometer tip on the device - a dilute bleach solution dripping into the mechanism quickly causes corrosion. The end-user cannot 're-calibrate' a GAT, but an end-user can verify whether the tonometer is within factory specifications using the calibration bar shipped with the device. The manufacturer recommends that this be done monthly, and that devices out of specification by more than ± 1 mmHg be returned for servicing. How often is this being done? Not often enough, it would seem. Kumar and Jivan (507) recently surveyed ophthalmology residents attending the Royal College of Ophthalmologists meeting, and found that 70% of respondents felt that verifying calibration of tonometers was someone else's responsibility and they did not know how often this was being done in their clinic. An earlier study, also performed in the UK, discovered that up to 50% of tonometers in community practice were out of specification by more than 2.5 mmHg!
So before we try to interpret our GAT measurements up or down a few mmHg because of thick or thin corneas, we should at least make sure that the underlying measurement is sound.
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